In simple terms, a rift can be considered as a fissure in the earth's surface that broadens over time; or more technically, as an elongate basin bounded by opposed steeply dipping normal faults. The East African Rift runs from the Afar Triple Junction in Ethiopia, southward through eastern Africa.
The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates called the Somalian Plate and the Nubian Plate, which are sub plates or protoplates. These two plates are moving away form each other and also away from the Arabian plate to the north. The point where these three plates meet in the Afar region of Ethiopia forms what is called a triple-junction.
The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift. Further to the South, a series of rifts occur which include a Western branch, the "Lake Albert Rift" or "Albertine Rift", which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-to-south, on a line slightly west of Nairobi (Figure 2). These two branches together have been termed the East African Rift (EAR), while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift (after the geologist who first mapped it in the early 1900's). The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS). The complete rift system therefore extends 1000's of kilometers in Africa alone; and several 1000’s more if we include the Red Sea and Gulf of Aden as extensions.
The stretching process associated with rift formation is often preceded by massive volcanic eruptions, which flow over vast areas and are usually preserved or exposed on the edges of the rift. Some geologists consider these eruptions to be "flood basalts", that is, the lava is erupted along fractures, rather than at individual volcanoes; and runs over the land in sheets like water does during a flood.
The East African Rift Zone comprises a number of both active and dormant volcanoes, including the following: Mount Kilimanjaro, Mount Kenya, Mount Longonot, Menengai Crater, Mount Karisimbi, Mount Nyairagongo, Mount Meru and Mount Elgon, including Crater Highlands in Tanzania.
The rifting of East Africa is complicated by the fact that two branches have emerged, one to the west which hosts the African Great Lakes, where the rift filled with water; and another nearly parallel rift about 600 kilometers to the east, which nearly bisects Kenya north-to-south before entering Tanzania where it seems to cease. Lake Victoria lies between these two branches. It is thought that these rifts are generally following old sutures between ancient continental masses that collided billions of years ago, to form the African craton; and that the split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. Considering that the rift could not penetrate this area, it instead diverged around it, leading to the two branches that can be seen today. The East African Rift System is an excellent field laboratory to study a modern, actively developing rift system.
This region is also imperative for grasping the roots of human evolution. The bones of several hominid ancestors of modern humans have been discovered there, including Lucy, a partial yet eye-opening australopithecine skeleton dating back to 3 million years. Some other hominid material recently discovered was found to be 10 million years old. The structure and evolution of the rift may have rendered East Africa more sensitive to climate changes, which led to many alternations between wet and arid periods.
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